Everyone else is posting their headcanons and whatnot, so me, the official number one Case Files fan, is here to yap
-> Boris ended up adopting Rosie in 1957, after the events of Death by Dolly, where she killed her parents and the police killed her uncle.
-> He took her home because when he rescued her from the police station "fire," they both quickly realized she didn't actually have anywhere to go. (Charlie took to having a 9 year old pseudo-daughter surprisingly quick, but then again he didn't know about any of the Legion and murder stuff)
-> She ran away in the early 60s, and they reunited in 2006 when he ended up in Carey due to an investigation and found her in the orphange, where she had befriended Cindy.
-> Murie wasn't killed after the totem incident, and instead Boris and the She-Devils became closer because of it, all of them having now been turned by the totems
-> Murie was later killed by Mary in 1958 as an act of revenge towards Boris in their never ending cycle of screwing each other over. Tim brought her back accidentally with the 1991 portal incident.
-> Boris can flirt with anyone for any reason, but the moment he tries to with Charlie he crumples like a wet receipt. That being said, he is incredibly smooth when flirting with Tim and Murie.
-> Boris founded Legendary Truth to help regular people with the supernatural side of their world, so that people wouldn't lose their loved ones to things they couldn't understand as often. Simultaneously, he was doing his best to subtly help different beings and creatures co-exist with mortal humans peacefully.
-> Calvin was one of the few people who knew about Boris being undead, which is why no one ever asked any questions about Boris being an active member of The Collective until he was 90 years old.
-> Do I remember what age I had him lie to be when LT started so no one asked questions about him not looking like he was the age he actually was? No. Does that only change the 90 to like 80 and not really matter specifically anyways? Yeah.
-> In case it wasn't obvious earlier, he is polyamorous as fuck, he's got a lot of love to give to his partners and nothing but time. Charlie is the most important one to him though. They get married when it becomes legal, and Boris wears the wedding ring on a necklace tucked under his shirt.
-> On a similar note, Tim and Murie eventually get married just because why not and Tim LOVES to call her his wife at every possible opportunity.
-> Boris and Mary went to college at Princeton together, and they dated and were considered the ideal couple, but they broke up due to having different life paths and drifted apart. While he could barely recognize her in '58, she had no recollection of him at all before dying.
-> Canonically, Charlie is two years older than Mary, and since Boris is her age, that does make Charlie the oldest between him, Boris, and Mary. (Boris is actually the youngest of the three, since Mary is an April birthday and he is a July birthday.) ((For the record, this also means Charlie is a Taurus, Mary is an Aries, and Boris is a Cancer.))
-> If Charlie or any of the patients die again post-2008, they end back up in the mirror again. Mary is usually very quick to boot them out if it does happen because she doesn't want Boris going after her again.
-> I mentioned that Boris and Mary have a never ending cycle of screwing each other over, but it does end after 2008 and they do slowly come to terms with co-existing again. She has forgiven him for killing her and him for her killing the patients because they both realized it was inevitable, however he still is pissed at her for the 50 years of suffering on both his and Charlie's behalf.
-> Charlie did spend the 50 years in the mirror in an endless loop of the events of Dead Exposure, albeit slightly everchanging depending on his actions, but his fate was always unescapable. Nothing he did would ever change that, and he lost count how many times he lived that apocalypse day.
-> Few quick rapid fire ones: Murie's last name is Schaeffer. Madam Basiel's (Boris' downstairs neighbor). Molly's (Boris' upstairs neighbor) last name is Kennedy. Molly and Alice are lesbians and in love. Rosie's full name was Rosaline Price before she became Rosie Shuster. Not even Boris knows how he got government funding for LT.
and that's it for now because I'm getting tired but I will properly elaborate on undead Boris later, but that's not a headcanon, that's a canon discussion and slight theory. :)